Massive collisions in the universe between black holes or dead stars appear to be at the higher end of estimates as, following the latest switching on of the three upgraded LIGO and Virgo detectors, scientists have detected gravitational waves emanating from the collision of two neutron stars, and another that could be the first evidence of neutron star-black hole collision.

"These two new triggers are further evidence that our universe regularly rings with the aftershocks of colossal astronomical events," said Professor Sheila Rowan, Director of the University of Glasgow's Institute for Gravitational Research. "We'd been deaf to those sounds before the detectors equipped us with the opportunity to hear them, and each event gives invaluable new data points to expand our understanding of our cosmos."

UK scientists and engineers play key roles in the construction and operation of the Laser Interferometer Gravitational-Wave Observatory (LIGO), which runs two detectors in the United States. A third detector named Virgo is operated by a European collaboration and is based in Italy.

On 25 April LIGO and Virgo detected gravitational waves from what appears to be a collision between two neutron stars about 500 million light-years from Earth. Neutron stars are the dense remnants of massive exploded stars. Just one day later, the network registered another event about 1.2 billion light-years away and initial analysis suggests it might have been the collision of a neutron star and black hole.

Professor Mark Hannam, Director of Cardiff University's Gravity Exploration Institute said: "Yet again the LIGO and Virgo detectors have surpassed expectations. Our most optimistic estimates were for a detection every week, and the first month of the run gave us five candidates."

Professor Alberto Vecchio, Director of the Institute of Gravitational Wave Astronomy, University of Birmingham, said: "LIGO-Virgo have got off to a flying start in the new observing run. We are busy following up several gravitational-wave detection candidates from binary systems of black holes and neutron stars. If the instruments continue to perform as they've done so far, it's going to be many sleepless nights of hard work to tease out from the data the full richness of information from these intriguing cosmic collisions."

While neutron star collisions cause gravitational waves, their impacts also release light across the electromagnetic spectrum. In 2017, LIGO-Virgo's first-ever detection of a gravitational wave from a neutron star collision was also observed by many conventional telescopes.

This time, telescopes around the world once again raced to track the sources and pick up the light expected to arise from these mergers. Hundreds of astronomers eagerly pointed telescopes at patches of sky suspected to house the signal sources. However, at this time, neither of the sources has been pinpointed.

These new results join the growing list of possible gravitational-wave detections since LIGO and Virgo resumed operations last month.

Professor Andreas Freise, Deputy Director, Institute of Gravitational Wave Astronomy, University of Birmingham said: "What a fantastic start! We had anticipated that LIGO and Virgo would observe many new signals during this observation run. It is very exciting to see nature providing us with several new signals in the first month already, fulfilling our earlier predictions."

The discoveries come just weeks after LIGO and Virgo were turned back on. The twin detectors of LIGO - one in Washington and one in Louisiana - along with Virgo, located at the European Gravitational Observatory (EGO) in Italy, resumed operations 1 April after undergoing a series of upgrades to increase their sensitivities to gravitational waves - ripples in space and time. Each detector now surveys larger volumes of the universe than before, searching for extreme events such as smash-ups between black holes and neutron stars.

Dr. Vivien Raymond, from Cardiff University's Gravity Exploration Institute, said: "LIGO-Virgo's third observing run has already proven to be more interesting than we expected, barely a month after it started. It's exciting to think about the next surprises for us to discover in the universe."

Related Links
Science And Technology Facilities Council In The UK
The Physics of Time and Space

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